1. Field of the Invention
The present invention relates to control transmissions with hydraulic and mechanical gearing, especially transmissions which are used in commercial utility vehicles.
2. Description of the Related Art
All gear shofts should be smooth under load, regardless of the type of engine or differential transmissions that are being applied. These requirement for a transmission control unit are best addressed with an electrohydraulic conyrol system, whose operating principle, for example, is described in the technical publication "Hydrodynamik in der Antriebstechnik", published by J. M. Voith GmbH, Vereinte Fachverlage Krauskopf-Ingenieur Digest, Mainz, 1987.
The electronics and hydraulics of the above-mentioned electrohydraulic control systems are joined together via solenoid valves including a magnetizing coil, a piston valve and a pull-back spring. Until now, the control of the solenoid valves has been accomplished by continuous controllers which were either hardware-based or software-based. A significant simplification in the control of automatic transmissions, as compared to the continuous controllers, was achieved by the application of two-step controllers (ref. Dubbel, reference book for mechanical engineering, Springer Verlag, Berlin, Heidelberg, N.Y., 1995, Pages X8 through X9). According to the state of the art, these controllers were built using hardware circuitry whereby, the comparators, for example, were applied in the form of connected operational amplifiers. The hardware-based circuitry was designed as application-specific integrated circuitry (ASIC). The setting of the hysteresis, which is required for the stable operation of a two-step controller (ref. Dubbel, rest as stated above, page X8), was accomplished by applying resistors into the circuit design. In a very time-consuming balancing process, these resistors had to be adjusted for every transmission control system as part of a post-manufacturing process. This required among other things, for example, the checking of the entire control system and a manual soldering of balance resistors into the circuitry.
Further, the hardware circuitry assigns a discrete manipulated output for every actual input value. This rigid relationship did not permit the control of two solenoid valves with one and the same controller.